Define a tie-trace in a binary phase diagram?

• A. The line showing the diffusion rate as a function of composition.
• B. A line indicating the end-point compositions of coexisting phases at a fixed temperature.
• C. A vertical line indicating constant temperature.
• D. The path that the overall composition follows as temperature changes, indicating which phases are present.

Answer: (D)

Tie-trace is about how the overall composition of a closed binary system evolves as temperature changes. For a given total composition, you heat or cool the material and, as you cross phase boundaries, the phase assemblage changes. The tie-trace follows that fixed overall composition through the phase diagram, showing which phases are present at each temperature (for example, moving from a single phase to a two-phase region and back as you pass boundaries).

This is different from a tie-line, which at a specific temperature connects the compositions of the two coexisting phases in equilibrium within a two-phase field. A vertical line at constant temperature isn’t describing the changing phase makeup with temperature, so it doesn’t capture the idea of the overall composition path—which is why the correct concept is the path of the overall composition as temperature changes.